5465
The Stereochemistry of Tris ( triphenylphosphine)rhodium Chloride Decarbonylation of Aldehydesls2 H. M. Walborsky” and L. E. Allen3
Contribution f r o m the Department of Chemistry, Florida State University, Tallahassee, Florida 32306. Received March 13, 1971 Abstract: A series of aldehydes differing in the hybridization of the carbon atom to which the carbonyl group is attached, i.e., (-)-(R)-2-methyl-2-phenylbutanal (2), (+)-(R)-l-methyl-Z,Z-diphenylcyclopropanecarboxaldehyde (3), and (E)-a-ethylcinnamaldehyde (4), was decarbonylated using tris(tripheny1phosphine)rhodium chloride (1). The products, (+)-(S)-2-phenylbutane (5), (+)-(S)-l-methyl-2,2-diphenylcyclopropane (6a), and (Z)-1-phenyl-1butene (7),were formed with 81, 94, and 100% retention of optical activity, respectively. The optical purity in the decarbonylation of a series of 1-substituted cyclopropyl aldehydes varied from 83 to 6%. A cleavage radical pair disproportionation mechanism is proposed for the reaction on the basis of the stereochemicalresults. The decarbonylation of 1-methyl-2,2-diphenylcyclopropanecarboxaldehyde-d was used to demonstrate the intramolecularity of the reaction as well as to illustrate its application to the synthesis of specifically deuterium-labeledcompounds.
In
connection with another problem, it became necessary for us to find a stereospecific method for the removal of carbon monoxide from an aldehyde. Although there are a number of methods available for decarbonylation of aldehydes4 most of them were not, nor did they show promise to be, stereospecific. It was only the decarbonylation of aldehydes by group VI11 metal compounds which seemed to offer this promise. These reactions were known for some time5 but it was the work of Tsuji and Ohno6 which called our attention to the synthetically useful application of this reaction with tris(tripheny1phosphine)rhodium chloride (1) as the reagent.
Several aldehydes have been decarbonylated using reagent 1. The reaction proceeds homogeneously in solvents such as benzene or dichloromethane in which 1 is moderately soluble. The reaction is sensitive to steric effects so that some aldehydes in which the carbonyl group is more hindered than a secondary alkyl carbon cannot normally be decarbonylated under the usual mild conditions. The more hindered aldehydes can, however, be smoothly decarbonylated using benzonitrile as the solvent.6 In order to evaluate the stereochemistry and stereoselectivity of the reaction, a number of aldehydes of known absolute configuration and varying in structure (1) The support of this work by grants from the National Science Foundation and Public Health Service Grant No. 04065 from the National Cancer Institute is gratefully acknowledged. (2) A preliminary account of this work has been reported: H. M. Walborsky and L. E. Allen, Tetrahedron Lett., 823 (1970). (3) In partial fulfillment of the requirements for the Ph.D. degree in Chemistry in the Graduate School of Syracuse University, July 1970. (4) For a review, see W. M. Schubert and R. R. ICintner in “The Chemistry of the Carbonyl Group,” S. Patai, Ed., Interscience, New York, N. Y., 1966, Chapter 14. ( 5 ) P. Sabatier and J. B. Senderens, C. R . Acad. Sci. Paris, 136, 738, 921 (1903); Y . J. Suen and S. Fan, J . Amer. Chem. Soc., 65, 1243 (1943); H. E. Eschinazi and H. Pines, J . Org. Chem., 24, 1369 (1959). ( 6 ) J. Tsuji and IC. Ohno, Tetrahedron Lett., 3962 (1965); K. Ohno and J. Tsuji, J . Amer. Chem. SOC.,90,99 (1968); for a recent review, see J. Tsuji and I(. Ohno, Synthesis, 157 (1969).
were decarbonylated. The aldehydes were selected so that the effect on the scope of the reaction of changing the hybridization of the carbon atom to which the carbonyl group is attached (sp3, sp2.28,sp2) could be evaluated. Syntheses and Absolute Configurations. The aldehydes selected for our study of the effect of change of hybridization were ( - )-(R)-2-methyl-2-phenylbutanal (2), ( +)-(R)-l-methyl-2,2-diphenylcyclopropanecarboxaldehyde (3a), and (E)-a-ethylcinnamaldehyde (4). The cyclopropyl system was selected to evaluate the effect of CY substituents on the stereochemical course of the decarbonylation. Instead of the methyl group in 3a, cyclopropyl systems were prepared, containing chloro (3b), fluoro (3c), and methoxy (3d) as substituents. The absolute configuration of (-)-(R)-2-methyl-2phenylbutanoic acid and (+)-(S)-2-phenylbutane (5) had previously been established’ and, therefore, in order to prepare aldehyde 2 of known configuration it was only necessary to convert (-)-(R)-2-methyl-2phenylbutanoic acid to aldehyde 2. This proved to be quite difficult and, of the many methods tried,8 only the reduction of the acid to the carbinol followed by oxidation with Jones reagentg in acetone produced the (-)-(R)-2-methyl-2-phenylbutanal (2) in acceptable yields (50-55 The syntheses and determination of the absolute configurations of the cyclopropyl aldehydes 3a-d and the cyclopropyl hydrocarbons 6a-d that would be derived from them by decarbonylation have previously been accomplished.lO~llChart I lists the relative and absolute configurational assignments.
z).
(7) D. J. Cram and J. Allinger, J . Amer. Chem. Soc., 76, 4516 (1594). (8) Attempts to reduce the corresponding acid chloride, N , N -
dimethylamide, and nitrile with a variety of aluminum hydrides failed. Oxidation of the carbinol with dicyclohexylcarbodimide and with acetic anhydride, or with lead tetraacetate, ceric ammonium nitrate, or chromic anhydride-pyridine, was also unsuccessful. (9) K. Bowden, I. M. Heilbron, E. R. H. Jones, and B. C. L. Weedon, J . Chem. Soc., 39 (1946). (10) H. M. Walborsky, L. Barash, A . E. Young, and F. J. Impastato, J . Amer. Chem. Soc., 83, 2517 (1961); H. M. Walborsky and C. G. Pitt, ibid., 84, 4831 (1962). (11) H. M. Walborsky, L. E. Allen, H.-J. Traenckner, and E. J. Powers, J . Org. Chem., 36, 2937 (1971).
Walborsky, Allen
Tris(tripheny1phosphine)rhodium Chloride
5466 Chart I 0
6a,
to 100% retention of configuration. Under more vigorous conditions using benzonitrile6 at 160” or no solvent and a catalytic amount of 1 at temperatures up to 260°, as much as 26% of the E-alkene 7 has been observed. In a blank experiment it has been shown (see Experimental Section) that the 2-alkene is converted to the E-alkene under the conditions of the reaction. A mechanism has been proposed for the decarbonylation of aldehydes6 which is similar to that proposed earlier for the decarbonylation of acid chlorides 6,14 The important intermediate is the acyl rhodium complex 8 which collapses to form hydrocarbon product, R-H, and complex 9. It is this step in the reaction on
X =CH,,(+)-(S)
b, X = C1, (+)-(S) c, X = F, (-)-(SI d, X = OCH,, (+)-(S)
1 C&\ H
/c=c
U
9
8
which we will focus our discussion. That the reaction is clearly intramolecular, that is, that the aldehyde hydrogen is the one which becomes bonded to the carbon atom from which the carbonyl is removed, is shown by the results of the decarbonylation of 1-methyl-2,2-diphenylcyclopropanecarboxaldehyde-~. When the aldehyde containing 96 f 3 % deuterium 0
/CH,CH3
H‘ (2).7
The configuration of (E)-a-ethylcinnamaldehyde (4) was established by oxidizing it to the known (E)-aethylcinnamic acid1*with silver oxide. The configuration of (Z)-l-phenyl-l-butene was assigned on the basis of nmr spectra in analogy to the work on the homologous 1-phenyl-l-pr~pene.’~The 2 isomer has a shorter retention time in glpc than the E isomer and the nmr spectra clearly distinguish the two isomers on the basis of the signals observed for the vinyl protons.13 The 2 isomer shows a pattern of six lines centered at 6 5.48 which appears to be a pair of triplets, whereas the E isomer shows a multiplet of four sharp lines centered at 6 6.08. Results and Discussion The decarbonylation of optically pure ( - ) - ( R ) - 2 required prolonged reaction at 160” in benzonitrile due most likely to the steric hindrance at the quaternary carbon atom. The sec-butylbenzene ( 5 ) isolated proved to be 81 % optically pure and to have the (+)-(S) configuration, which corresponds to an overall retention of configuration. The decarbonylation of the cyclopropyl aldehyde (+)-(R)-3a proceeded smoothly in refluxing xylene to give a 70 % yield of (+)-(S)-6a of 94 % optical purity and retained configuration. The Evinyl aldehyde 4, when decarbonylated in refluxing benzene, produced only the 2-alkene 7, corresponding (12) N. E. Hoffman, A. T.Kanakkanatt, and R. F. Scheider, J . Org. Chem., 27, 2687 (1962). (13) J . A. Pople, W. G. Schneider, and H. J. Bernstein, “High
Resolution Nuclear Magnetic Resonance,” McGraw-Hill, New York, N. Y., 1959, pp 238-240.
at the aldehyde carbon is decarbonylated in xylene with 1, the product hydrocarbon contains 93 f 3z deuterium in the 1 position. This result is quite significant in that it demonstrates the synthetic utility of the decarbonylation reaction in the synthesis of specifically deuterated hydrocarbons. This mild procedure will make it possible to prepare hitherto inaccessible optically active deuterated hydrocarbons. The high stereoselectivity observed and the lack of rearranged products militate against the incursion of a cationic alkyl intermediate as the migrating species. For if an alkyl cation was involved, one would have observed extensive racemization in 5 as well as ringopened hydrocarbon in the decarbonylation of the cyclopropyl aldehyde 3a. l 5 Based on our early results2 a carbanionic intermediate seemed most attractive. Cram16 has shown that in cleavage reactions generating a tetrahedral carbanion the result is high retention of configuration. Although vinyl anions have apparently not been generated in cleavage reactions, the ability of vinyllithium compounds to maintain their configuration is well documented.l6 This also holds equally true for the cyclopropyl anion such as the l-methyl-2,2-diphenylcyclopropyl anion which when observed as the (14) J. Blum, E. Oppenheimer, and E. D. Bergmann, J . Amer. Chem. SOC.,89, 2338 (1967). (15) C. H. DePuy, L. G. Schnack, J . W. Hanser, and W. Wideman, ibid., 87, 4006 (1965); P. v. R. Schleyer, D. W. van Diene, U. SchoIIkopf, and J. Paust, ibid., 88, 2868 (1966). (16) D. J. Cram, “Fundamentals of Carbanion Chemistry,” Academic Press, New York, N. Y., 1965, p 153.
Journal of the American Chemical Society / 93.21 / October 20, 1971
5467 lithium," sodium,18 or Grignardlg reagent or when generated as the leaving group in Haller-Bauer cleavage reaction*O has been shown to be configurationally stable.
anionic
radical p a i r
Since the reaction has been shown to be intramolecular (uide supra), a free-radical mechanism can also accommodate the stereochemical results. An intramolecular reaction can, in a formal sense, be viewed as a cage reaction. Recent work has shown that radical cage disproportionationz1 as well as cage recombination reactions2 can occur with a high degree of stereoselectivity. The work of LansburyZ3 and Schollkopfz4is a case in point. These authors have provided evidence that the Wittig rearrangement, which had previously been assumed to be an anionic reaction, involved not anions but radical pairs. Moreover, it should be noted that in the rearrangement of benzyl 2-phenylbutyl ether, the migrating 2-phenylbutyl radical involved in the cleavage-recombination reaction maintained its configuration to the extent of -90% ( 4 0 % optical purity). This compares favorably to our observation on the decarbonylation of (-)(R)-2 in which the (+)-(S)-2-phenylbutane formed was of retained configuration with an optical purity of 81 %. The radical pair mechanism becomes even more attractive when one examines the results of our study on the effect of LY substituents on the stereochemistry of the decarbonylation of the cyclopropyl aldehydes 3a-d, Table I. As can be seen the per cent of Table I. Decarbonylation of Optically Active 1-Substituted 2,2-Diphenylcyclopropanecarboxaldehydes
% 1 substituent
optical purities are calculated by using the maximum rotations of 6b-d which were obtained by the HallerBauer sodium amide cleavage" of their 1-benzoyl precursors. The Haller-Bauer reaction has been shown to proceed in a stereospecific manner1' and to involve carbanion intermediates.l1P2j The stereoselectivity observed in the decarbonylation of aldehydes with tris(tripheny1phosphine)rhodium chloride (1) is consistent with the radical pair mechanism proposed.
Config ([CyIHp, deg)"
Product, config ([CylH., deg)
optical purityC
a Experiments were performed on incompletely resolved samples and corrected. Enantiometer was used. Based on optical purities previously determined for 6b, 6c, and 6d (ref 11).
optical purity varies from 83% for the 1-chloro compound 3b to 6 for the 1-methoxyl derivative 3d. The (17) H. M. Walborsky, F. J. Impastato, and A. E. Young, J . Amer. Chem. Soc., 86, 3283 (1964). (18) H . M. Walborsky and J. B. Pierce, J . Org. Chem., 33, 1962 ( 1968).
(19) H . M. Walborsky and A. E. Young, J . Amer. Chem. Soc., 86, 3288 (1964). (20) F. J. Impastato and H. M. Walborsky, ibid., 84, 4838 (1962). (21) H. M. Walborsky and J.-C. Chen, ibid., 93, 671 (1971). (22) P. D. Bartlett and J. D. McBride, Pure Appl. Chem., 15, 89 (1967); I
